1. Field of the Invention
This invention relates to a differential mechanism of the type used in axle differentials and those formed integrally with manual and automatic transmissions.
2. Description of the Prior Art
U.S. Pat. No. 4,221,138 describes a differential housing having a full cubic form comprising five parts formed from sheet metal. The carrier of the pinion gears has a square cross section. Two cover assemblies having integral journals for the stub shafts are assembled from both sides to the pinion carrier. These units are joined together with a driving gear by four bolts and nuts. The differential housing described in U.S. Pat. No. 5,533,423 is fabricated from deep drawn steel parts including a conical bearing jacket carrying a pinion shaft and pinions, and a cover carrying a bevel gear closes an opening in the jacket.
The basic function of a differential mechanism is to transmit torque from a transmission output to axle shafts, which allow the right and left driven wheels to rotate at different speeds. Various differential mechanisms have been developed, the simplest and most frequently used being the bevel gear differential.
A bevel gear differential consists of two or four pinion gears, depending on the weight of the vehicle and the magnitude of torque to be transmitted. The pinions, often called planetary gears, are supported together with the two side bevel gears in the differential housing or housing.
Torque is applied from the final drive of the transmission to a ring gear, for front wheel driven vehicles, or from the drive pinion to a crown wheel, for rear wheel driven vehicles. The ring gear and crown wheel are joined to the differential housing, from which rotation is transmitted to a pinion shaft that supports the pinions. The pinion gears distribute the applied torque equally to the side gears, which drive the axle shafts and the wheels. When the vehicle negotiates a turn, a speed difference between the inner and outer wheels occurs, causing the pinion gears to rotate. Because of this additional rotation, the speed of the outer wheel is increased to the same extent that the speed of the inner wheel decreases.
Conventional housings for differential mechanisms are made from a one-piece iron casting. Iron castings have significantly lower ductility and yield strength than most steels; therefore, they require thick walls to supplement their relatively low strength. The greater thickness produces increased weight and increases the moment of inertia, which reduces the efficiency of the powertrain. Frequently in conventional differential housings, two windows in laterally opposite walls are required to allow assembly of internal bevel gears and pinions. This arrangement limits the torque capacity of the housing due to the reduction in stiffness and strength caused by the presence of the windows.
Iron castings require a large number of machining operations to produce finished parts because the dimensions of the parts as cast cannot be controlled closely due to shrinkage during the transition from the liquid phase to the solid phase. Currently a conventional differential mechanism can only be assembled manually due to poor internal accessibility and the number of different joining axes. Machining requires an expensive, space-intensive transfer line having low flexibility.
Correct operation and vibration-free motion of the entire differential depends on accurate geometry of all matching components and stiff support of the gears by the differential housing. The differential housing not only carries the internal gears and ring gear or ground wheel, but also carries the differential bearings and, in many housings, the speedometer gear also. These components require different geometric features to ensure their accurate location support and centering.
Iron castings have several material and technology-relate inefficiencies that limit their use in some differential housings. Ductility and yield strength of nodular cast iron are significantly lower than those of steel. Consequently they result in a heavier housing than would be required if the housing were made of steel.